The Ehrlichia are obligate intracellular pathogens that infect circulating white blood cells in mammalian hosts. Ehrlichia canis can infect canines and humans and cause canine monocytic ehrlichiosis (CME) and human monocytic ehrlichiosis (HME), respectively. The canine disease is characterized by fever, lymphadenopathy, weight loss, and pancytopenia. In humans the disease is characterized by fever, headache, mylagia, and leukopenia. Early detection and treatment are important for treating both canine and human ehrlichiosis.